1. Field of the Invention
The invention relates to system and method of reproducing an image, and more particularly to a system and method of reproducing an image having a large number of intensity levels, on a device capable of producing a limited number of intensity levels.
2. Description of the Related Art
Many image reproduction systems do not render faithful or pleasing reproductions of continuous tone originals. For example, in many developable image copiers, the typical discharge characteristic of the photoconductor and solid area developability of the image development system combine to yield a Tone Reproduction Curve (TRC) with a steep slope and a narrow range. The result is a copy with washed out highlights and overdeveloped shadows. A standard method used to improve the gray scale reproduction is to expose the photoconductor through a transmissive screen to produce a fine, spatially modulated voltage pattern on the photoconductor. Development of this modulated pattern yields a Tone Reproduction Curve with a lower slope and an extended range of input gray scale that produces a corresponding change in the output. The result is more faithful and pleasing copy.
FIG 1. shows another method of presenting gray levels to the viewer in a typical reproduction system. The image to be reproduced is partitioned into mutually exclusive areas called "halftone cells," each containing a number of pixels. The size of the sample area of the input section of the reproduction system corresponds to a halftone cell, while the size of the printing area in the output section of the typical reproduction system corresponds to a pixel. Gray levels are simulated by turning on a subset of the total number of pixels in a halftone cell in an output section of the reproduction system. The pixels are too small for the viewer to perceive the printing of individual pixels, and the viewer instead perceives a gray level corresponding to the percentage of pixels that are turned on.
FIG. 1 shows seventeen different halftone cells corresponding to the range of gray levels between black (0) and white (16). To print black no pixels in the halftone cell are turned on as shown at portion 105 in FIG. 1. To simulate an intermediate level of gray for the printing of a certain cell, half the pixels might be turned on as shown at level eight, portion 110 in FIG. 1.
A fundamental drawback of the scheme illustrated in FIG. 1 is a trade-off that exists between the number of gray levels and resolution. Since the area of the input sample is not the area of printing resolution, a pixel, but is instead the larger halftone cell area, resolution is reduced. Reducing the size of the halftone cell to include fewer pixels will increase resolution while reducing the number of gray levels. Halftone methods employed in developable image printers have traditionally been binary, that is, the laser writes with only two laser intensity levels; on and off.
U.S. Pat. No. 4,196,454 discloses a method of halftoning that tends to enhance the resolution of edges in the reproduced image, given a certain halftone cell size. FIG. 2 shows a halftone cell portioned into areas corresponding to the pixels of the cell with each area having an associated intensity threshold value. In operation, where the pixel input intensity is equal to or above the screen threshold value for a given pixel position, a binary 0 output results. Where the pixel voltage level is below the screen threshold value, a binary 1 output results. This operation is shown in FIGS. 3 and 4. FIG. 3 is an exemplary input cell where the pixels each happen to have a corresponding intensity value of 49 FIG. 4 is the binary output pattern resulting from the processing of the pixels in FIG. 3 with the thresholds shown in FIG. 2. This method of processing is sometimes called "binary screening."
U.S. Pat. No. 4,868,587 discloses a system for increasing the number of gray levels given a certain halftone cell size. In U.S. Pat. No. 4,868,587, the average value of the input pixel intensities is used to access a lookup table to select an appropriate halftone cell to be printed. Where it is desired to preserve image microstructure such as edges to enhance resolution, pixels within the halftone cell are rearranged during processing so as to best correspond to the position of the original pixels in the cell of pixels being processed. White pixels in the selected halftone cell selected are relocated to match the location of the whitest pixel in the input halftone cell, black pixels in the selected halftone cell are relocated to match the location of the blackest pixel in the input halftone cell, light gray pixels in the selected halftone cell are relocated to match the location of the light gray pixels in the input halftone cell, etc.